Allow Vite dev server to access fixture files

[marlo-longley]

Currently our integration/demo page at http://localhost:4444/sequence-switching.html is broken, because it relies on an internal manifest URL:

mirador/__tests__/integration/mirador/mirador-configs/multiple-sequences.js

Line 8 in 15b6015

manifestId: 'http://localhost:4444/__tests__/fixtures/version-2/multipleSequences.json',

This is currently broken.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.1%. Comparing base (5af3981) to head (3bd69bf).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4212   +/-   ##
=====================================
  Coverage   95.1%   95.1%           
=====================================
  Files        326     326           
  Lines      16192   16192           
  Branches    2546    2547    +1     
=====================================
  Hits       15391   15391           
  Misses       796     796           
  Partials       5       5           

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